Phase addressed holographic associative memory

ABSTRACT

The present invention provides a holographic storage apparatus comprising a polarizing beam splitter configured to split an incoming beam into an object beam and a reference beam; a first spatial light modulator configured to modulate the object beam with an array of data; a second spatial light modulator configured to phase modulate the reference beam with an orthogonal phase function; a holographic medium configured to record an interference pattern between the modulated object beam and the modulated reference beam; a first image sensor configured to read an image of the modulated object beam; and a second image sensor configured to read an image of the modulated reference beam.

CROSS-REFERENCE TO RELATED, APPLICATIONS

This application is a continuation of and claims the benefit of U.S.application Ser. No. 12/559,416 filed Sep. 14, 2009, which is herebyincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to data storage, and moreparticularly, some embodiments relate to phase addressed holographicmemory.

DESCRIPTION OF THE RELATED ART

The development of Holography was pioneered in the mid 20th century byphysicist Dennis Gabor, who ultimately received a Nobel Prize in Physicsfor his achievements. The advent of the laser in 1960 allowed advancesused in modern holographic techniques. The first holograms that recorded3D objects were made in 1962 by Yuri Denisyuk in the Soviet Union, andby Emmett Leith and Juris Upatnieks at the University of Michigan, inthe United States of America.

FIG. 1 is a diagram illustrating a simple example of a conventionalholographic technique. Referring now to FIG. 1, in this example thecollimated laser light beam 102 is provided for the holographic process.This beam can be provided, for example, by a laser light source. Thebeam splitter 104 splits the beam into 2 components to achieve an objectbeam 105 and a reference beam 107. As illustrated in FIG. 1, object beam105 impinges on an object 108 to be imaged. Light 103 reflected byobject 108 impinges on a recording medium 109. Reference beam 107 isfolded by a folding mirror 106 onto recording medium 109. Theinterference between reference beam 107 and object beam 103 results in apattern of varying intensity that is captured by recording medium 109.

Once the pattern is captured or recording medium 109, it can berecovered by illuminating the recording medium 109 with a reproductionof the original reference beam 107. In other words, the light fielddiffracted by the reference beam impinging on recording medium 109 isidentical to the light field that was originally scattered by the object108 that was imaged. Various materials can be used for recording medium109 including, for example, photographic film, photoresists, photopolymers, and so on.

Holography, however, is not limited to applications of a photographicnature. Indeed, with the ever-increasing demand for high data-storagecapacities in small volumes, developers have turned to holography as amechanism for achieving high-density data storage. Configurationssimilar to that shown in FIG. 1 can be utilized to achieve high-density,high-speed data storage through holographic techniques. For example,high-density high-speed holographic memory has been proposed for avariety of storage applications. High-density high-speed holographicmemory technologies have been developed for onboard storage for bothweapons and test instrumentation systems. One example system useselectro-optic beam-steering technology to provide 2-D recording. Usingthis technology, two one-dimensional beam-steering spatial lightmodulators can be cascaded in orthogonal configuration to arrive at a2-dimensional angular-fractal multiplexing scheme for 2-D recording. Anexample of using a beam steering spatial light modulator for holographicrecording is described in U.S. Pat. No. 7,149,014, which is incorporatedherein by reference in its entirety.

To increase recording density, light models of 2-dimensional Walshfunctions have been used in either or both of the object and referencefields to achieve deep 3-D holographic recording. In such aconfiguration, different objects, can be recorded on a hologram so thateach of them is recorded with one of the Walsh function reference beams.Accordingly, on reading the hologram using one of the Walsh functions,the object recorded with that Walsh function can be reconstructed.Examples of using Walsh functions to achieve multiplexed recording isdescribed in U.S. Pat. Nos. 5,940,514, and 5,627,664, which areincorporated herein by reference in its entirety.

BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION

According to various embodiments of the invention, systems and methodsare provided for holographic data storage. In particular, in accordancewith some embodiments of the invention, orthogonal functions arecombined with two-dimensional holographic recording to achieve 3dimensional recording in a holographic medium. In one implementation, a2-dimensional spatial light modulator is used to allow imaging of a2-dimensional array or “page” of data on a holographic medium. This canbe combined with a 2nd 2-dimensional spatial light modulator toimplement orthogonal function such as, for example, a Walsh function, toachieve 3-dimensional recording of multiple pages from the 2-dimensionalspatial light modulator.

Cubic beam splitters can be used in conjunction with two-dimensionalspatial light modulators to direct the object and reference beams onto arecording medium such as a photo refractive crystal. Image sensors suchas, for example, CCD or CMOS image sensors can be used to capture imagesrecorded on the holographic medium for readout, transfer, ageprocessing, or other purposes.

Although the holographic memory can be used in the number ofenvironments or applications and built to a wide array of varyingspecifications, in one application a holographic memory can be used foradvanced massive onboard data storage for satellites, aircraft, weaponssystems, and other applications. Applications of the holographic memoryare not limited to onboard data storage and can be used in otherenvironments as well. Depending on the materials and configurationselected and implemented, high capacities of 16 terabytes or greater canbe achieved with a low volume and weight, as low as 100 cm cubed, 500 g,for example, and some applications may approach a petabyte capacity in a1 cubic cm recording medium. Likewise, some implementations may achievehigh survivability-on the order of 35 to 50 G.

Embodiments of the invention can be implemented utilizing anon-scanning, orthogonal binary phase reference beam to address a3-dimensional holographic memory for data recording and retrievaloperations.

According to an embodiment of the invention, a holographic storageapparatus comprises a polarizing beam splitter configured to split anincoming beam into an object beam and a reference beam; a first spatiallight modulator configured to modulate the object beam with an array ofdata; a second spatial light modulator configured to phase modulate thereference beam with an orthogonal phase function; a holographic mediumconfigured to record an interference pattern between the modulatedobject beam and the modulated reference beam; a first image sensorconfigured to read an image of the modulated object beam; and a secondimage sensor configured to read an image of the modulated referencebeam.

Other features and aspects of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, which illustrate, by way of example, the featuresin accordance with embodiments of the invention. The summary is notintended to limit the scope of the invention, which is defined solely bythe claims attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention, in accordance with one or more variousembodiments, is described in detail with reference to the followingfigures. The drawings are provided for purposes of illustration only andmerely depict typical or example embodiments of the invention. Thesedrawings are provided to facilitate the reader's understanding of theinvention and shall not be considered limiting of the breadth, scope, orapplicability of the invention. It should be noted that or clarity andease of illustration these drawings are not necessarily made to scale.

FIG. 1 is a diagram illustrating a simple example of a conventionalholographic technique.

FIG. 2 is a diagram illustrating an example holographic recording systemin accordance with one embodiment of the invention.

FIG. 3A illustrates an example set of orthogonal binary phase patterns(Walsh functions, in this example).

FIG. 3B illustrates an example for generating an orthogonal functionoptically by using a commercially available phase-only spatial lightmodulator in accordance with one embodiment of the invention.

FIG. 4 is a high-level block diagram illustrating a control system forcontrolling a holographic recording system such as that describedherein.

FIG. 5 is a diagram illustrating another example implementation of aholographic recording system in accordance with one embodiment of theinvention.

FIG. 6 is a diagram illustrating a perspective view of a holographicrecording system in accordance with one embodiment of the invention.

FIG. 7 is a diagram illustrating an example system package fordeployment in accordance with one embodiment of the invention.

FIG. 8 illustrates an example computing module that may be used inimplementing various features of embodiments of the invention.

The figures are not intended to be exhaustive or to limit the inventionto the precise form disclosed. It should be understood that theinvention can be practiced with modification and alteration, and thatthe invention be limited only by the claims and the equivalents thereof.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The present invention is directed toward a system and method forproviding a management system for holographic data storage. Inparticular, in accordance with some embodiments of the invention,orthogonal function reference beams are combined with two-dimensionalholographic recording to achieve three-dimensional recording in aholographic medium. In one implementation, a 2-dimensional spatial lightmodulator is used to allow imaging of a 2-dimensional page of data on aholographic medium. This can be combined with a 2nd 2-dimensionalspatial light modulator to apply in orthogonal phase function such as,for example, a Walsh function, to achieve 3-dimensional recording ofmultiple pages from the 2-dimensional spatial light modulator.

From time-to-time, the present invention is described herein in terms ofexample embodiments, environments and applications. Description in termsof these embodiments, environments and applications is provided to allowthe various features and embodiments of the invention to be portrayed inthe context of an exemplary scenarios. After reading this description,it will become apparent to one of ordinary skill in the art how theinvention can be implemented in different and alternative embodiments,environments and applications.

FIG. 2 is a diagram illustrating an example holographic recording systemin accordance with one embodiment of the invention. Referring now toFIG. 2, a laser 132 is provided to generate a coherent beam of light,which is used to create the object beam 150 and the reference beam 158.In one embodiment, a compact diode-pumped solid-state 532 nm Nd:YAGlaser with feedback control to maintain wavelength and power stabilityand with intensity level control can be used. In this exampleimplementation, a half wave plate 134 is provided to control thepolarization of the laser light.

The half wave plate can be manually controlled or an electronicallycontrolled (for example computer-controlled) half wave plate can be usedto allow remote or electronic adjustment. Half wave plate 134 may beused to control the relative power between the object beam 150 and thereference beam 158. As described below, object beam 150 is formed bytransmission of an incoming beam through polarizing beam splitter 145,while reference beam 158 is formed by reflection off of polarizing beamsplitter 145. Accordingly, by adjusting the polarity of the incomingbeam using half wave plate 134, the intensity—and therefore the power—ofthe two beams may be adjusted. In further embodiments, the half waveplate 134 may be used during data reading operations. As describedherein, data retrieval may comprise two operations. In the firstoperation, a reference beam used to record a particular data page isdetermined by transmitting a partially modulated object beam throughholographic medium 169. In the second operation, the determinedreference beam is transmitted through the holographic medium 169 toretrieve the entire data page. Accordingly, at any given step duringthis retrieval operation, only one beam is required. The half wave plate134 may therefore be used to provide these single beams. When an objectbeam is needed (for reference reconstruction), the half wave plate 134may be used to adjust the incoming polarity for total transmissionthrough the polarizing beam splitter 145. When a reference beam isneeded (for data page retrieval), the half wave plate 134 may be used toadjust the incoming polarity for total reflection by the polarizing beamsplitter 145.

A shutter 136 is also provided in the illustrated example to allowexposure to be controlled and the beam to be momentarily suspended fromread/write operations without the need to power the laser 132 up anddown. This can be useful, for example, to update spatial lightmodulators 152, 168, between pages of data.

Objective lens 138 can be provided along with a spatial filter 140 toachieve the desired beam filtering. A collimating lens 143 provides acollimated beam for object beam 150 and reference beam 158. Polarizingbeam splitter 145 splits the collimated beam into object beam 150 andreference beam 158. Particularly, polarizing beam splitter 145 transmitsobject beam 150 at a 1st polarization, and reflects reference beam 158at a 2nd polarization. Adjusting half wave plate 134 alters the polarityof the light beam, which can be useful for adjusting the relative energyof the transmitted and reflected beams at polarizing beam splitter 145.

In the illustrated example, object beam 150 passes through shutter 148and half wave plate 151 before reaching polarizing beam splitter 155.Polarizing beam splitter 155 directs the object beam 150 to spatiallight modulator 152. Half wave plate 151 can be used to adjust thepolarization of object beam 150 to optimize reflection at polarizingbeam splitter 155 according to the desired system parameters. Forexample, inherent characteristics of spatial light modulator 152 mayintroduce a polarization rotation in object beam 150. Half wave plate151 can compensate for this rotation by pre-rotating the polarization ofobject beam 150 in the opposite direction as the spatial light modulator152. Spatial light modulator 152 modulates reflected object beam 152creating amplitude modulated object beam 154 and directs modulatedobject beam 154 toward holographic recording medium 169. Polarizing beamsplitter 155 is configured to transmit waves that are polarizedorthogonally to the optical plane and to reflect waves having that arepolarized along the optical plan in the direction transmitted bypolarizing beam splitter 145. Accordingly, the entire object beam 150 isfirst reflected onto spatial light modulator 152 and then transmittedsuch that it is orthogonal to the optical plane when incident on theholographic medium 169.

A lens 160 is included to focus the pixels from spatial light modulator152 onto the corresponding location on the data CMOS or CCD sensor array172. In one embodiment, holographic recording medium 169 is implementedusing a thick or volume holographic medium such as for example anFe:LiNbO₃ crystal holographic recording material, although otherphotorefractive crystals or recording materials can be used. Thickphotopolymer recording materials can be used for 90° recording with highBragg selectivity. The specific polymerization process of thesematerials can be optimized in various embodiments for low scatter in athick volume of photopolymer medium. Thick recording materials generallyhave a thickness greater than a wavelength or two of that of the objectand reference beams.

Reference beam 158 is directed through shutter 149 and reflected bypolarizing beam splitter 164 onto phase-only spatial light modulator 168after passing through half wave plate 144. Phase spatial light modulator168 is used to modulate reference beam 158 with an orthogonal functionsuch as, for example, a Walsh function or other orthogonal function. Themodulated reference beam 159, after passing back through the half waveplate 144 is directed through polarizing beam splitter 164 and focusedby a lens 162 onto holographic recording medium 169. Polarizing beamsplitter 164 is configured to transmit waves that are polarizedorthogonally to the optical plane and to reflect waves having that arepolarized in the direction reflected by polarizing beam splitter 145.Accordingly, the entire reference beam 158 is first reflected ontospatial light modulator 168 and then transmitted such that it isorthogonal to the optical plane when incident on the holographic medium169. Because both the reference beam 159 and object beam 154 are inparallel polarization orthogonally to the optical plane, internalBrewster Bragg effects are avoided, thereby increasing the efficiency ofthe holograms, and resolution and storage capacity of the holographicmedium 169.

Modulating reference beam 158 with an orthogonal function such as aWalsh function allows multiple object images from object spatial lightmodulator 152 to be recorded onto holographic recording medium 169.Because the Walsh function modulates the reference beam with anorthogonal pattern, multiple images or pages of data can be recorded,and each image retrieved using a reference beam modulated with itsassociated orthogonal pattern. FIG. 3A is a diagram illustrating justone example of 16 frames of an orthogonal Walsh function that can beused to modulate the reference beam 158. In various embodiments,geometry of the binary phase patterns for the orthogonal function can bedetermined based on a tradeoff between the correlation signal andreadout data page signal.

CCD, CMOS or other image sensors 170, 172 can be used for data andreference image retrieval and readout. Images can be captured by imagesensors 170, 172 in real time as data is being recorded or when an imageis being retrieved. Image sensors 170, 172 can be interfaced with imageprocessing and capture electronics to capture retrieved images.Commercially available high-resolution (>1024×1024) high-speed CCD orCMOS image sensors can be used for correlation signal and data pagereadout. CCD modules with thermoelectric (TE) cooling are preferable forenhanced sensitivity and speed. A high-speed frame-grabber can beincluded to interface to a PC or other computing system for furtherimage capture, transfer and processing. For example, connections suchas, for example, IEEE 1394 (FireWire) and SATA II connections can beused.

In the illustrated example, hologram multiplexing and addressing areperformed using collinear reference beams, which form an orthogonal setof binary phase-encoded 2D array patterns, where the number oforthogonal patterns, N, can be very large (for an n×n array,N=n!/((n/2)!)²). Therefore, multiplexibility is greatly increased. Insuch embodiments, multiplexibility theoretically needs to only belimited by the maximum achievable index modulation of the recordingmaterial. In these examples, the hologram is recorded as the spatialinterference pattern between a binary phase-encoded reference beam fromphase spatial light modulator and an object beam from the data spatiallight modulator. The data on the data spatial light modulator representsa 2D bit-map (“page”) of data. Such recording can be repeated at thesame location, i.e., multiplexed, by using other binary-phase patternsbelonging to the same orthogonal set. As an example, using a phasespatial light modulator with 32×32 pixels (i.e., n=32), there areN=32!/(16!)²=6×10⁸, number of holograms. Assuming a 1024×1024 pixel datapage (or 1 Megapixel data spatial light modulator), with the 32×32 pixelphase spatial light modulator, a data capacity of 6×10¹⁴ bits or 600Tbits (75 Tbytes) can be achieved in a 1 cm³ crystal.

Example embodiments with phase-encoded 2D reference beam and parallelholographic correlation search method can be implemented to avoid strictlimitations on the number of recorded holograms, and can be implementedto access hundreds of terabits of stored data in milliseconds. Theexample embodiment illustrated herein uses a symmetric 90° recording andretrieving geometry and parallel holographic correlation search. Suchdesigns can be implemented without requiring any movement of therecording medium. The unique 90° holographic recording geometry can alsoaid in achieving higher capacity, since it allows optimal filtering ofthe noise signal and provides a high signal-to-noise ratio in theholograms (limited only by the pixel contrast ratio of the data spatiallight modulator and CMOS/CCD).

Additionally, the highly parallel holographic associative search with apartial image data key (i.e., context addressing mode) allowssimultaneous retrieval of correlated signals corresponding to possibledata pages of which the key forms a part. In this case, the correlationsignals can be processed by real-time software to identify the rightdata image and retrieve the full data page from memory in a fewmilliseconds. For example, a portion of a page of data can be used torecreate or identify the phase pattern that was used to modulate thereference beam. With the particular orthogonal function identified, thepage of memory to which the data segment belongs can then be identified,and the data retrieved.

With a 1024×1024 pixel data page with an update speed of 10 kHz,achievable with current spatial light modulators (SLMs) such as theTexas Instruments Digital Micromirror Device (DMD), a data transfer rateof 10 gigabit/s is achievable.

As stated above, the holographic recording system can be integrated witha computing system for image retrieval, readout, analysis, and otherimage operations. As also stated above, operation of the holographicrecording system can be controlled by computing system. This can be thesame computing system used for image processing, or separate computercontrol can be provided. FIG. 4 is a high-level block diagramillustrating a control system for controlling a holographic recordingsystem such as that described herein. Referring now to FIG. 4, theillustrated example includes a computing system 212 that is used tocontrol electronics 214. Computing system 212 can be implemented usingany of a number of computing systems or control modules including, forexample, a personal computing system configured to interface to controlelectronics 214. Likewise, computing system 212 and control electronics214 can be implemented utilizing a dedicated control system. Controlelectronics 214 in the illustrated example includes electronics utilizedto actuate a plurality of drivers 215 to control componentry such as alaser, shutters, half wave plates, spatial light modulators, imagecapture devices, and so on. Thus, a control system like computing system212 can be used to coordinate shutter operations with the refresh of thedata in phase spatial light modulators for example.

FIG. 5 is a diagram illustrating another example implementation of aholographic recording system in accordance with one embodiment of theinvention. Referring now to FIG. 5, this example illustrates a 90°configuration similar to that shown in the example of FIG. 2. However,in this example, the polarization beam splitters 232, 234, 236 and theholographic recording medium 238 are shown in a more tightly packedconfiguration along with spatial light modulators 235, 237 CCDs 241,242, and lenses 244, 245, 246. Also shown are a Nd:YAG(Neodymium:Yttrium, Aluminum Garnet) laser 261 and/or a helium neon(HeNe) laser 262 coupled through half wave plates 263, shutters 264, andpolarizing beam splitters 265 through a lens 266 to generate the objectand reference beams. In this example configuration, the laser can beselected based on the type of crystal employed. In one embodiment, thepolarizing beam splitters 232, 234, 236 are cubic configuration suchthat when they are stumbled with a cube of photo refractive crystal usedfor the holographic recording medium, the system can be packaged andcontained in a confined cubic arrangement. Examples of this arediscussed in more detail below with reference to FIGS. 6 and 7.

In various embodiments, such as system can be configured in a compacthigh density cubic memory system. The data storage system can usenonscanning, orthogonal binary phase reference beam to address thememory for recording and retrieval purposes, and the use of orthogonalfunctions can be employed to provide three-dimensional (3D) holographicmemory.

FIG. 6 is a diagram illustrating a perspective view of a holographicrecording and retrieval system in accordance with one embodiment of theinvention. Referring now to FIG. 6, in this example implementationillustrates holographic recording media 325 which includes, for example,beam splitters lenses and the holographic recording crystal such as, forexample, that illustrated in FIG. 5. This perspective view illustratesan example of how these components can be configured in a cubic or blockarrangement and illustrates an example of their spatial relation tospatial light modulators 326, 327 and CCD/CMOS image sensors 329, 331.As this example illustrates, a data input spatial light modulator 326provides an image (representing data or an object 334, for example) tobe recorded in a frame or page of the holographic storage medium withinrecording media 325. This example also illustrates a spatialrelationship between recording media 325 spatial light modulators 326,327, and CCD image sensors 329, 331.

FIG. 7 is a diagram illustrating an example system package fordeployment in accordance with one embodiment of the invention. Thisexample illustrates three cubic polarizing beam splitters 437 arrangedadjacent to the holographic storage medium, which in this example is aphoto refractive crystal 438. This example also illustrates oneembodiment for arrangement of data spatial light modulator positioned inproximity to the beam splitter/crystal arrangement, and an arrangementfor a data CCD image sensor 443, and a correlation CCD image sensor 444.In this example, the laser 446, optics 448 and electronics 447 arehoused in the casing 450 just below the beam splitter/mirror/crystalarrangement. Although not illustrated, control and image processingelectronics can also be included within housing 450, as can appropriatepower sources. However, other embodiments, utilize external processingcapabilities and power sources. Using external control, processing, andpower functionality can allow the recording module to be implemented ina small volume, low weight configuration. Accordingly, multiplerecording modules can be interfaced to a single control and processingsystem to increase storage capacity.

Although not illustrated, heat spreaders can be added to the package toallow more uniform heat distribution across the components, therebyreducing thermal effects of the system. Index matching andantireflective coatings layers can be used to improve the opticalperformance of the system where multiple components are integrated suchas being splitters, lenses, heat spreaders, and other opticalcomponents. Were necessary or useful, and are reflective coatings canalso be utilized.

Although the components of a storage module such as those illustrated inthe examples above can be scaled according to the desired specificationsand applications, one example system is now described. As would beapparent to one of ordinary skill in the art after reading thisdescription other components and arrangements can be utilized to achievedesired system size, power consumption and resolution parameters. Inthis example, the recording module utilizes small cubes of polarizingbeamsplitters on the order of 1-1.5 mm³. The beam splitters can be gluedtogether with high speed 1-4 kHz frame rate, high-resolution imagesensors such as, for example, 1024×1024 reflective, CMOS-backplaneferroelectric spatial light modulators for a 2-D data or other objectinput. In this example, a medium resolution (e.g., 128×128) high-speed(1-4 kHz frame rate) orthogonal binary phase-only CMOS-backplane FLC-SLMcan be used for phase array reference beam generation. The image sensorcan be implemented using two high-resolution (1024×1024) high-speed (1-4kHz) CMOS or other CCD sensor arrays. One can be used for 2-D pageread-out and the other for correlation signal detection with associatedmemory read-out. The laser can be implemented using a moderate power (<1W) compact green laser at 532 nm (e.g., a frequency doubled Nd:YAGsolid-state laser) for both hologram recording and read-out.Additionally an additional low-power HeNe laser diode may be includedfor two-photon rewritable hologram recording). This example can also usean approximately 1 cm cube suitably doped lithium niobate (LiNbO3)crystal for 90°-geometry volume hologram recording. Flat holographicoptical element (HOE) lenses can be used for laser beam collimation andfor spatial-light modulator to CMOS/CCD imaging, along with thinpolarization optics (½λ and ¼λ plates) and shutters for object andreference beams. The spatial-light-modulator and CMOS drivers andcontrol electronics can also be integrated into a compact (4.5×4.5×3cm≈60 cm³) volume inside a ruggedized and thermally controlled (EMIshielded) metallic module. The input and output data are connected via astandard high-speed/parallel data bus interconnection for interfacing toexisting instrumentation.

The configuration into solid cube optics with no air gaps provides ahighly stable configuration supported by optically gluing the componentsand mounting in a strengthened and hardened by mechanical frame. Inaccordance with this example, the entire module can be on the order of4.5 cm×4.5 cm×3 cm. With a glass density of ˜2.5 g/cm³, the opticalmodule in such an example will weigh less than 100 g. With the laser andelectronics as well as the mechanical casing and connectors, the totalweight of the module in such an example configuration can weigh lessthan 500 g.

Using a single laser, both recording and readout of the holograms can bemade via compact (˜1 mm flat) HOE lenses, with 1:1 image relationshipbetween the input data spatial light modulator to the readout imagesensor array, and the reference beam spatial light modulator to thecorrelation image sensor array. This symmetric configuration allows themodule to have a cubic monolithic structure. Using such a configuration,a holographic recording system can be implemented to have a highsurvivability rate for onboard applications. Further, because of the useof solid-state laser and CMOS-backplane spatial light modulators andsensors, the electrical power consumption can be small (<10 watts),allowing the system to operate with standard 12 V/24 V DC onboard powersources.

Embodiments that perform recording and read out using a collinear binaryphase encoded orthogonal reference wave, can be implemented to takeadvantage of the concept that an arbitrary wavefront of an object can bedecomposed into plane waves, each of which is recorded and read outindependently. That is, a wavefront that is the sum of N plane waves canbe represented as N orthogonal eigenfunctions, i.e., modes φ_(n). Eachof these modes can be made to pass independently through a 3D hologramas if through an optically uniform medium and does not change its form.In mathematical terms, this means that a self-conjugate operator Hhaving N eigenfunctions φ_(n), can be compared to a 3D hologram. Whenreading out with a specific eigenfunction (which has been used as areference wave in multiple recording), orthogonality nullifies all thereconstructed object waves except the one used in recording with thatreference wave.

Square array phase patterns can form an orthogonal at of plane waves ifthe total area of positive (phase 0) and negative (phase π) regions areequal. FIG. 3A illustrates an example set of orthogonal binary phasepatterns (Walsh functions, in this example). FIG. 3B illustrates anexample for generating an orthogonal function optically by using acommercially available spatial light modulator in accordance with oneembodiment of the invention. One example of a spatial light modulator isa Binary Phase only spatial light modulator from Boulder NonlinearSystems, Inc., or from Brillian Corp.).

Hologram multiplexing using collinear phase-encoded speckle referencewaves can be used, but may result in noise (e.g., due to crosstalk) anda need for critical registration in the read out. These drawbacks aredue to the use of a random set (rather than an orthogonal set) ofreference waves. In preferred embodiments, using a large thickness ofthe holograms and the consequent high angular selectivity, the twoadjacent regions on a phase pattern are recorded independently withoutsubstantial crosstalk.

Some embodiments are configured to perform multiplexing and addressingwith collinear (e.g., same direction) reference beams, which form anorthogonal set of binary phase-encoded 2D array patterns. The number oforthogonal patterns, N, can be very large (for an n×n array,N=n!/((n/2)!)²). Therefore, multiplexibility can be high, but may belimited by the maximum achievable index modulation of the material. Thehologram can be recorded as the spatial interference pattern between abinary phase-encoded reference beam from the phase spatial lightmodulator and an object beam from the data spatial light modulatorrepresenting a 2D bit-map (“page”) of data. With orthogonal referencefunctions, the recording can be repeated at the same location, i.e.,multiplexed, by using other binary-phase patterns belonging to the sameorthogonal set. Using a phase spatial light modulator with 32×32 pixels(i.e., n=32), N=32!/(16!)²=6×10⁸, number of holograms can be achieved.With a 1024×1024 pixel data page (˜10⁶ bits), a data capacity of 6×10¹⁴bits or 600 Tbits (75 Tbytes) in 1 cm³ crystal can be achieved. Withhigher resolution spatial light modulators, even greater storagecapacities can be achieved.

As previously noted, embodiments of the invention use a symmetric 90°recording and retrieving geometry and parallel holographic correlationsearch without requiring any movement of the recording medium. The 90°holographic recording geometry allows improved filtering of the noisesignal and provides a high signal-to-noise ratio in the holograms overother configurations.

Additionally, embodiments can be implemented to utilize a highlyparallel holographic associative search with a partial image data key(i.e., context addressing mode). This allows simultaneous retrieval ofall correlated signals corresponding to possible data pages of which thekey forms a part. In this case, the correlation signals are processed inreal-time to identify the right data image and retrieve the full datapage from memory in a few milliseconds. For example, with a 1024×1024pixel data page with an update speed of 10 kHz, achievable with currentspatial light modulators (SLMs) such as the Texas Instruments DigitalMicromirror Device (DMD), a data transfer rate of 10 gigabit/s isachievable.

In one embodiment, the system is configured as a modular system suchthat components can be removed/replaced. For example, in one embodimentthe recording crystal can be removed/inserted. This modular nature ofthe recording medium facilitates replacement of the media as well astransfer of the data.

Various embodiments described above utilized a nonscanning orthogonalphase reference-beam addressing technique that allows a plurality ofdifferent reference beams, and thus a corresponding number of 2Dhologram pages, to be stored in the volume of the recording crystal.Accordingly, embodiments of the invention can be implemented so as toovercome limitations of standard angular multiplexed (scanned)holographic recording in which the maximum number of hologram pages arelimited by the angular resolution and the 2D surface area of therecording crystal. For example, with a 1024×1024≈10⁶ binary pixelspatial light modulator, and 10⁶ or 10⁸ reference codes, the total datastorage capacity can reach 10⁶×10⁶ or 10⁶×10⁸ or 10¹² to 10¹⁴terabits/cm³.

The orthogonal binary phase array pattern preferably uses o, π phases toprovide orthogonality. With currently available spatial lightmodulators, the Walsh functions or other phase codes can be generated athigh-speed (1-4 kHz) so that 2D data can be accessed at 1 to 4 Gbits/srate. The use of a phase-coded reference read-out beam can also providesecured access to the stored data, as the data cannot be retrievedwithout access to the orthogonal functions used to write the data.

Control and other electronics described herein can be implementedutilizing any form of hardware, software, or a combination thereof. Forexample, one or more processors, controllers, ASICs, PLAs, PALs, CPLDs,FPGAs, logical components, software routines or other mechanisms mightbe implemented to make up a module. In implementation, the variousmodules described herein might be implemented as discrete modules or thefunctions and features described can be shared in part or in total amongone or more modules. In other words, as would be apparent to one ofordinary skill in the art after reading this description, the variousfeatures and functionality described herein may be implemented in anygiven application and can be implemented in one or more separate orshared modules in various combinations and permutations. Even thoughvarious features or elements of functionality may be individuallydescribed or claimed as separate modules, one of ordinary skill in theart will understand that these features and functionality can be sharedamong one or more common software and hardware elements, and suchdescription shall not require or imply that separate hardware orsoftware components are used to implement such features orfunctionality.

Where components or modules of the invention are implemented in whole orin part using software, in one embodiment, these software elements canbe implemented to operate with a computing or processing module capableof carrying out the functionality described with respect thereto. Onesuch example computing module is shown in FIG. 8. Various embodimentsare described in terms of this example-computing module 500. Afterreading this description, it will become apparent to a person skilled inthe relevant art how to implement the invention using other computingmodules or architectures.

Referring now to FIG. 8, computing module 500 may represent, forexample, computing or processing capabilities found within desktop,laptop and notebook computers; hand-held computing devices (PDA's, smartphones, cell phones, palmtops, etc. mainframes, supercomputers,workstations or servers; or any other type of special-purpose orgeneral-purpose computing devices as may be desirable or appropriate fora given application or environment. Computing module 500 might alsorepresent computing capabilities embedded within or otherwise availableto a given device. For example, a computing module might be found inother electronic devices such as, for example, digital cameras,navigation systems, cellular telephones, portable computing devices,modems, routers, WAPs, terminals and other electronic devices that mightinclude some form of processing capability.

Computing module 500 might include, for example, one or more processors,controllers, control modules, or other processing devices, such as aprocessor 504. Processor 504 might be implemented using ageneral-purpose or special-purpose processing engine such as, forexample, a microprocessor, controller, or other control logic. In theillustrated example, processor 504 is connected to a bus 502, althoughany communication medium can be used to facilitate interaction withother components of computing module 500 or to communicate externally.

Computing module 500 might also include one or more memory modules,simply referred to herein as main memory 508. For example, preferablyrandom access memory (RAM) or other dynamic memory, might be used forstoring information and instructions to be executed by processor 504.Main memory 508 might also be used for storing temporary variables orother intermediate information during execution of instructions to beexecuted by processor 504. Computing module 500 might likewise include aread only memory (“ROM”) or other static storage device coupled to bus502 for storing static information and instructions for processor 504.

The computing module 500 might also include one or more various forms ofinformation storage mechanism 510, which might include, for example, amedia drive 512 and a storage unit interface 520. The media drive 512might include a drive or other mechanism to support fixed or removablestorage media 514. For example, a hard disk drive, a floppy disk drive,a magnetic tape drive, an optical disk drive, a CD or DVD drive (R orRW), or other removable or fixed media drive might be provided.Accordingly, storage media 514 might include, for example, a hard disk,a floppy disk, magnetic tape, cartridge, optical disk, a CD or DVD, orother fixed or removable medium that is read by, written to or accessedby media drive 512. As these examples illustrate, the storage media 514can include a computer usable storage medium having stored thereincomputer software or data.

In alternative embodiments, information storage mechanism 510 mightinclude other similar instrumentalities for allowing computer programsor other instructions or data to be loaded into computing module 500.Such instrumentalities might include, for example, a fixed or removablestorage unit 522 and an interface 520. Examples of such storage units522 and interfaces 520 can include a program cartridge and cartridgeinterface, a removable memory (for example, a flash memory or otherremovable memory module) and memory slot, a PCMCIA slot and card, andother fixed or removable storage units 522 and interfaces 520 that allowsoftware and data to be transferred from the storage unit 522 tocomputing module 500.

Computing module 500 might also include a communications interface 524.Communications interface 524 might be used to allow software and data tobe transferred between computing module 500 and external devices.Examples of communications interface 524 might include a modem orsoftmodem, a network interface (such as an Ethernet, network interfacecard, WiMedia, IEEE 802.XX or other interface), a communications port(such as for example, a USB port, IR port, RS232 port Bluetooth®interface, or other port), or other communications interface. Softwareand data transferred via communications interface 524 might typically becarried on signals, which can be electronic, electromagnetic (whichincludes optical) or other signals capable of being exchanged by a givencommunications interface 524. These signals might be provided tocommunications interface 524 via a channel 528. This channel 528 mightcarry signals and might be implemented using a wired or wirelesscommunication medium. Some examples of a channel might include a phoneline, a cellular link, an RF link, an optical link, a network interface,a local or wide area network, and other wired or wireless communicationschannels.

In this document, the terms “computer program medium” and “computerusable medium” are used to generally refer to media such as, forexample, memory 508, storage unit 520, media 514, and channel 528. Theseand other various forms of computer program media or computer usablemedia may be involved in carrying one or more sequences of one or moreinstructions to a processing device for execution. Such instructionsembodied on the medium, are generally referred to as “computer programcode” or a “computer program product” (which may be grouped in the formof computer programs or other groupings). When executed, suchinstructions might enable the computing module 500 to perform featuresor functions of the present invention as discussed herein.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not of limitation. Likewise, the various diagrams maydepict an example architectural or other configuration for theinvention, which is done to aid in understanding the features andfunctionality that can be included in the invention. The invention isnot restricted to the illustrated example architectures orconfigurations, but the desired features can be implemented using avariety of alternative architectures and configurations. Indeed, it willbe apparent to one of skill in the art how alternative functional,logical or physical partitioning and configurations can be implementedto implement the desired features of the present invention. Also, amultitude of different constituent module names other than thosedepicted herein can be applied to the various partitions. Additionally,with regard to flow diagrams, operational descriptions and methodclaims, the order in which the steps are presented herein shall notmandate that various embodiments be implemented to perform the recitedfunctionality in the same order unless the context dictates otherwise.

Although the invention is described above in terms of various exemplaryembodiments and implementations, it should be understood that thevarious features, aspects and functionality described in one or more ofthe individual embodiments are not limited in their applicability to theparticular embodiment with which they are described, but instead can beapplied, alone or in various combinations, to one or more of the otherembodiments of the invention, whether or not such embodiments aredescribed and whether or not such features are presented as being a partof a described embodiment. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” shouldbe read as meaning “including, without limitation” or the like; the term“example” is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; the terms “a” or“an” should be read as meaning “at least one,” “one or more” or thelike; and adjectives such as “conventional,” “traditional,” “normal,”“standard,” “known” and terms of similar meaning should not be construedas limiting the item described to a given time period or to an itemavailable as of a given time, but instead should be read to encompassconventional, traditional, normal, or standard technologies that may beavailable or known now or at any time in the future. Likewise, wherethis document refers to technologies that would be apparent or known toone of ordinary skill in the art, such technologies encompass thoseapparent or known to the skilled artisan now or at any time in thefuture.

The presence of broadening words and phrases such as “one or more,” “atleast,” “but not limited to” or other like phrases in some instancesshall not be read to mean that the narrower case is intended or requiredin instances where such broadening phrases may be absent. The use of theterm “module” does not imply that the components or functionalitydescribed or claimed as part of the module are all configured in acommon package. Indeed, any or all of the various components of amodule, whether control logic or other components, can be combined in asingle package or separately maintained and can further be distributedin multiple groupings or packages or across multiple locations.

Additionally, the various embodiments set forth herein are described interms of exemplary block diagrams, flow charts and other illustrations.As will become apparent to one of ordinary skill in the art afterreading this document, the illustrated embodiments and their variousalternatives can be implemented without confinement to the illustratedexamples. For example, block diagrams and their accompanying descriptionshould not be construed as mandating a particular architecture orconfiguration.

1. A holographic storage apparatus, comprising: a polarizing beam splitter configured to split an incoming beam into an object beam and a reference beam; a first spatial light modulator configured to modulate the object beam with an array of data; a second spatial light modulator configured to phase modulate the reference beam with an orthogonal phase function; a holographic medium configured to record an interference pattern between the modulated object beam and the modulated reference beam; and a image sensor configured to read an image of the modulated object beam or an image of the modulated reference beam.
 2. The apparatus of claim 1, wherein the first spatial light modulator is configured to amplitude modulate the object beam with the array of data.
 3. The apparatus of claim 2, wherein the data array is a two dimensional array of data and the reference beam is a two dimensional orthogonal phase function.
 4. The apparatus of claim 3, wherein the first spatial light modulator is configured to modulate a light beam with a portion of the array of data and wherein the image sensor is configured to read an image formed when the modulated light beam impinges on the holographic medium and further comprising a processor configured to determine the orthogonal phase function used to record the hologram with the modulated object beam.
 5. The apparatus of claim 4, wherein the first and second spatial light modulators are reflection spatial light modulators and further comprising: a second polarizing beam splitter configured to reflect the object beam onto the first spatial light modulator and to transmit the modulated object beam; a third polarizing beam splitter configured to reflect the reference beam onto the second spatial light modulator and to transmit the modulated reference beam.
 6. The apparatus of claim 5, wherein the first polarizing beam splitter, the second polarizing beam splitter, and the third polarizing beam splitter comprise solid optic components.
 7. The apparatus of claim 6, wherein the first polarizing beam splitter is fixedly coupled to the second polarizing beam splitter and the third polarizing beam splitter and the holographic medium is fixedly coupled to the second polarizing beam splitter and the third polarizing beam splitter such that the first polarizing beam splitter, the second polarizing beam splitter, the third polarizing beam splitter, and the holographic medium form quadrants of a rectangular prism; wherein the first spatial light modulator is fixedly coupled to a first face of the second polarizing beam splitter and the first sensor is fixedly coupled to a first face of the holographic medium that opposes the first face of the second polarizing beam splitter; and wherein the second spatial light modulator is fixedly coupled to a first face of the third polarizing beam splitter and a second image sensor is fixedly coupled to a second face of the holographic medium that opposes the first face of the third polarizing beam splitter.
 8. The apparatus of claim 7, further comprising a heat spreader configured to distribute heat.
 9. The apparatus of claim 1, wherein the modulated object beam and the modulated reference beam are polarized in a direction orthogonal to an optical plane when forming the interference pattern.
 10. The apparatus of claim 1, wherein the holographic medium comprises an erasable holographic medium, and a further comprising a laser configured to generate an erasing light beam configured to erase the erasable holographic medium.
 11. A method for holographic data storage and retrieval, comprising: performing a holographic data storage method comprising: modulating an object beam with a two dimensional array of data; modulating a first reference beam with a two dimensional orthogonal phase function; recording an interference pattern formed between the modulated object beam and the modulated reference beam in a holographic medium; and performing a holographic data retrieval method comprising: modulating a light beam with a portion of the two dimensional array of data to determine the orthogonal phase function used to record the modulated reference beam; and modulating a second reference beam with the determined orthogonal phase function to determine the two dimensional array of data.
 12. The method of claim 11, wherein the object beam is amplitude modulated with the two dimensional array of data.
 13. The method of claim 12, wherein the method of data storage further comprises: splitting an incoming beam using a first polarizing beam splitter to form the object beam and the reference beam; reflecting the object beam off of a first spatial light modulator to modulate the object beam; and reflecting the reference beam off of a second spatial light modulator to modulate the reference beam.
 14. The method of claim 13, further comprising adjusting the polarity of the incoming beam to adjust power of the reference beam and to adjust the power of the object beam.
 15. Computer executable program code embodied on a computer readable medium configured to cause a holographic storage apparatus to perform the functions of: directing an incoming beam at a first polarizing beam splitter configured to split an incoming beam into an object beam and a reference beam; directing the object beam to reflect off of a first spatial light modulator to modulate the object beam with a two dimensional array of data; directing the reference beam to reflect off a second spatial light modulator to phase modulate the reference beam with an orthogonal phase function; recording an interference pattern formed between the modulated object beam and the modulated reference beam; recording an image of the modulated object beam formed when the modulated reference beam is transmitted through the holographic medium; and recording an image of the modulated reference beam formed when a light beam modulated with a portion of the two dimensional array of data is transmitted through the holographic medium.
 16. The computer executable program code of claim 15, wherein the first spatial light modulator is configured to amplitude modulate the object beam with the array of data.
 17. The computer executable program code of claim 16, further configured to cause the apparatus to perform the function of adjusting the polarity of the incoming beam to adjust the intensity of the reference beam and to adjust the intensity of the object beam.
 18. The computer executable program code of claim 16, further configured to cause the apparatus to direct the object beam to a second polarizing beam splitter configured to reflect the object be onto the first spatial light modulator and to transmit the modulated object beam; and to cause the apparatus to direct the reference beam to a third polarizing beam splitter configured to reflect the reference beam onto the second spatial light modulator and to transmit the modulated reference beam.
 19. The computer executable program code of claim 18, wherein the first polarizing beam splitter, the second polarizing beam splitter, and the third polarizing beam splitter comprise solid optic components.
 20. The computer executable program code of claim 19, wherein the first polarizing beam splitter is fixedly coupled to the second polarizing beam splitter and the third polarizing beam splitter and the holographic medium is fixedly coupled to the second polarizing beam splitter and the third polarizing beam splitter such that the first polarizing beam splitter, the second polarizing beam splitter, the third polarizing beam splitter, and the holographic medium form quadrants of a rectangular prism; wherein the first spatial light modulator is fixedly coupled to a first face of the second polarizing beam splitter and the first image sensor is fixedly coupled to a first face of the holographic medium that opposes the first face of the second polarizing beam splitter; and wherein the second spatial light modulator is fixedly coupled to a first face of the third polarizing beam splitter and the second image sensor is fixedly coupled to a second face of the holographic medium that opposes the first face of the third polarizing beam splitter. 